1. Field of the Invention
The present invention relates to a method to prevent a delayed fracture of a steel member, and relates to a method for setting a shape and working stress, and working environment of a steel member so as to prevent a delayed fracture. More specifically, the present invention relates to a method for setting a shape and working stress, and working environment of a steel member so as to prevent a delayed fracture in steel members using high strength steel or the like having tensile strength of more than 1,000 MPa.
2. Description of the Related Art
Recently, high strength steel having tensile strength of more than 1,000 MPa has been developed. As it is known that a delayed fracture is found from such high strength steel, the steel is rarely employed for practical use. The delayed fracture is a phenomenon where high tensile steel suddenly fractures due to stress at less than tensile strength after a certain period.
Even though the same stress is applied to steel members, some members fracture and others do not due to a difference in stress concentration depending on the shape of the steel members. Thus, steel members have been conventionally designed to prevent stress concentration for the steel members from which a delayed fracture is expected. However, it is practically impossible to completely prevent a delayed fracture simply by changing the shape of steel members. Thus, steel members in complex shapes, such as bolts, have been designed with an extremely high safety factor.
Another method to prevent a delayed fracture is to increase Hc, in other words, a maximum value of diffusible hydrogen contents of unfailed specimens. However, a delayed fracture has not yet been prevented completely by the method.
An object of the present invention is to solve the above-noted conventional problems. The purposes of the present invention include: accurate prediction of a delayed fracture of a steel member, complete prevention of a delayed fracture by appropriately setting the shape, working stress and working environment of a steel member, and designing even a steel member in a complex shape with an appropriate safety factor.
In order to solve the above-noted problems, a method for setting a shape and working stress of a steel member according to the present invention includes the steps of finding the relationship between a maximum value of diffusible hydrogen contents of unfailed specimens (Hc) and Weibull stress, and finding the content of diffusible hydrogen entered into steel from the environment due to corrosion during the use of the steel member (He); finding Weibull stress in the Hc which is equal to the He; and determining a shape and working stress of the steel member so as to provide stress below the Weibull stress.
It is preferable that the Weibull stress is calculated by an electronic arithmetic unit in accordance with a finite element method based on the following Formula 1:                               σ          w                =                              [                                          1                                                      V                    0                                    ⁢                                      xe2x80x83                                                              ⁢                              xe2x80x83                            ⁢                                                ∫                                      V                    f                                                        xe2x80x83                                                  ⁢                                                                            (                                              σ                        eff                                            )                                        m                                    ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                                          V                      f                                                                                            ]                                1            m                                              Formula  1            
A method for setting a working environment of a steel member according to the present invention includes the steps of finding the relationship between a maximum value of diffusible hydrogen contents of unfailed specimens (Hc) and Weibull stress ("sgr"wA), and finding Weibull stress ("sgr"wB) of the steel member in actual use based on a shape and working stress of the steel member in actual use; finding Hc in the Weibull stress ("sgr"wA) which is equal to the Weibull stress ("sgr"wB); and determining a working environment of the steel member so as to provide the content of diffusible hydrogen entered into steel from the environment due to corrosion during the use of the steel member (He) which is less than the Hc.
It is preferable that the Weibull stress ("sgr"wA) and the Weibull stress ("sgr"wB) are calculated by an electronic arithmetic unit in accordance with a finite element method based on the Formula 1 mentioned above.
A method for evaluating a delayed fracture of a steel member includes the steps of finding the relationship between a maximum value of diffusible hydrogen contents of unfailed specimens (Hc) and Weibull stress ("sgr"wA), and finding the content of diffusible hydrogen entered into steel from the environment due to corrosion during the use of the steel member (He); finding the Weibull stress ("sgr"wA) in the Hc which is equal to the He; and comparing the xcfx81wA to Weibull stress ("sgr"wB) which is calculated from a shape and working stress of the steel member in use, and determining that there will be no delayed fracture when the "sgr"wA is larger than the "sgr"wB.
The present invention is based on the following knowledge obtained by the present inventors.
(1) The occurrence of a delayed fracture can be principally predicted based on the Weibull stress ("sgr"w) and a maximum value of diffusible hydrogen contents of unfailed specimens (Hc). In other words, the occurrence of a delayed fracture can be predicted without depending on a shape and working stress of a steel member.
(2) When a maximum value of diffusible hydrogen contents of unfailed specimens (Hc) is larger than the content of diffusible hydrogen entered into steel from the environment due to corrosion during the use of the steel member (He), a delayed fracture will not occur.